In the ultrafast electronics and optoelectronics communities, especially in the sub-field of applied terahertz beams, the detection of freely propagating picosecond microwave and millimeter-wave signals is primarily being carried out via photoconductive antennas and far-infrared interferometric techniques. For example, reference an article by Hu and Nuss entitled "Imaging With Terahertz Waves," Optics Letters, Vol. 20, No. 16 (August 1995).
Photoconductive antennas have good detection responsivity, and their signal-to-noise ratios are typically far better than liquid helium cooled bolometers. Further, the detection bandwidth of a photoconducting antenna with a short dipole length can exceed 5 THz. However, the limitation of these antenna-based detectors is the resonant behavior of their Hertzian dipole structure. This type of structure has a resonant wavelength at twice the dipole length and therefore the signal waveform, which includes the resonant detector response function, is not a simple cross-correlation of the incoming terahertz and optical gating pulses. Even if the temporal resolution of photoconductive antennas, which is limited by the finite lifetime of photo carriers in the optical gate and antenna geometry, is reduced below 100 fs, the measured signal will still not provide an accurate representation of the actual terahertz waveform.
In comparison, although far-infrared interferometric techniques provide an autocorrelation of terahertz pulses, important phase information is still lost. In most field-matter interconnection applications, knowledge of the entire terahertz waveform, including both the amplitude and phase, is crucial. Thus, to support a variety of advanced scientific and technological applications, there continues to exist a need for the development of more suitable sensing devices.
An electro-optic sampler is especially suitable for measurement of picosecond transient signals. Such samplers have been applied in the art for "local field" measurement, including measurement of signals produced by photodiodes, integrated circuits and other fast devices which either have an electrical stimulus and electrical output or an optical stimulus and an electrical output. These "local field" electro-optic sampling systems, such as described in U.S. Pat. Nos. 4,618,819, 4,910,458 and 5,406,194, typically utilize Pockels effect. A Pockels cell comprises what is referred to as an electro-optic crystal which has the property of variable birefringence as a function of electrical field applied thereto.
The electro-optic crystal is utilized in the "local field" context as follows: an optical pulse train is provided from a source and split into two different paths, a sampling beam and a stimulus beam. One such source is a visible wavelength picosecond laser. Optical pulses in the first path trigger generation of the electrical signal to be measured. This electrical signal is coupled to be accessible to the electro-optic crystal, through which optical sampling pulses of the second path are propagated. The crystal is in an optical path between first and second crossed polarizers. The field-induced birefringence varies the polarization of the sampling beam. The sampling beam intensity after polarization analysis is measured by a detector, for example, a slow photodiode, one which does not have to resolve individual pulses.
The detector output is provided to utilization means. Electrical output from the detector as well as electrical output indicative of modulation of pulses in the stimulus beam are first coupled to a lock-in amplifier which yields a dc output proportional to the amplitude of the sampled electrical signal in phase with the modulation of the stimulus beam. A display can be generated by plotting the output of the lock-in amplifier during successive pulse periods against the output of a variable delay line synchronized with the display device. The basic theory of electro-optic sampling is explained in Vladmanis and Mourou, "Electro-Optic Sampling: Testing Picosecond Electronics," Laser Focus/Electro-Optics, p. 84, February, 1986, and Vladmanis, Mourou and Gabel, IEEE Journal of Quantum Electronics, Vol. QE-19, 4, p. 664, April 1983. An effective electro-optic sampler for measuring signals having temporal components on the order of picosecond is disclosed in U.S. Pat. No. 4,446,425 issued to Vladmanis and Mourou.
In the most common implementation of electro-optic sampling, the electro-optic sampler is embodied in a test fixture composed of three parts. These are a metal or ceramic carrier, a photoconductive switch and an electro-optic crystal. The carrier provides mechanical support for active devices. The active devices include the electro-optic crystal itself, the photoconductive switch and the device-under-test. Electrical connections are made from the device-under-test to the waveguides in the switch and on the crystal as well as to a bias network typically with gold wire bonds.
In the operational mode, the photoconductive switch has appropriate bias supplied thereto. When it is stimulated with the stimulating beam described above, an electrical pulse with picosecond rise time is launched down the waveguide. This is the stimulus signal which stimulates or turns on the device-under-test. The device-under-test produces an electrical output pulse which is then launched down the waveguide on the crystal surface where its electrical field effects the birefringence of the electro-optic crystal and is sampled by the second train of optical pulses.
Although achieving good performance for quantifying "local field" characteristics, electro-optic sampling as known in the art and summarized herein, has heretofore been unworkable for free-space radiation characterization. This is principally because of the different natures of local field and free-space electromagnetic waves.
Thus, there exists a need in the art for a practical electro-optic sampling apparatus and method capable of sampling free-space radiation, and particularly to one which is suitable for real-time two-dimensional far-infrared imaging applications.